Project Summary/Abstract The long-term goals of this project are to understand how motion information from the inner-ear balance sensors of the vestibular system contributes to the control of balance in humans and to determine the extent to which galvanic vestibular stimulation (GVS) can be used to restore a useful vestibular contribution to balance in subjects with diminished vestibular function. GVS has long been used to demonstrate qualitatively a vestibular contribution to balance control, but a detailed quantitative understanding is lacking. Engineering system-identification methods and modeling will be applied to develop GVS as a quantitative tool for investigating the vestibular contribution to balance control and as a potential prosthetic aid to enhance balance in subjects with vestibular deficits. The proposed work has three specific aims. The first aim will combine experimental and modeling methods to: (1) define how motion information from vestibular sensors contributes to balance control, (2) determine how vestibular orientation information is combined with information from proprioceptive, somatosensory, and visual systems, (3) determine how combined sensory information is used to generate corrective responses to external perturbations imposed on a multi-segmental body, and (4) identify how subjects with bilateral vestibular loss (BVL) compensate for their vestibular loss. Models developed in the first aim provide quantitative hypotheses about mechanisms controlling balance. The second aim will use results from GVS tests to identify aspects of GVS-evoked body sway that differ from sway expected from the natural activation of vestibular receptors by actual head motion. We will develop a method that accounts for these differences, and then use GVS to test whether our model-based hypotheses of multi-segmental balance control predict how the vestibular contribution to balance control changes as a function of environmental and stimulus conditions. The third aim will use the identified characteristics of GVS-evoked balance responses and a model-based understanding of the balance control system to develop a method that uses GVS feedback, based on real-time measures of head motion, to manipulate the vestibular contribution to balance control. Because many BVL subjects remain sensitive to GVS, the potential exists for real-time GVS feedback to restore a functionally useful vestibular contribution to balance control in BVL subjects. Successful completion of the proposed experiments will provide new insights into the vestibular contribution to human balance control and mechanisms that compensate for vestibular loss. The development of methods for accurately manipulating vestibular-motion information using GVS feedback will facilitate future investigations of the vestibular contribution to motor tasks and could potentially contribute to the development of a vestibular prosthesis.